The present invention relates to fluidized bed reactors of the type including a fluidization zone separated from an air chamber by a grid structure provided with an outlet opening for coarse material.
Coarse, non-inflammable material which must be removed is gathered in the lower part of the fluidized bed reactor. Discharging this coarse material from a reactor is problematic because:
the temperature of the material to be discharged is high, i.e., 700.degree. to 1000.degree. C. at which temperature the strength of the construction materials is low.
the bed material to be discharged contains both fine particles and coarse material. The aim is to discharge mainly the coarse material, e.g., ash, inert bed material, masonry and iron pieces that have come into the bed material with the fuel material or from the combustor itself. As the temperature and the particle size of the bed material to be discharged varies greatly, the flow properties of the material also change.
the amount of the material to be discharged from the reactor may exceed 50% of the input fuel flow whereby heat recovery becomes essential when considering the efficiency of the boiler.
In U.S. Pat. No. 3,397,657, there is disclosed an apparatus for discharge of coarse material. This apparatus comprises a pipe connected with an outlet opening in the grid. The operation of the apparatus is controlled by an adjustable baffle supplied in the inclined portion of the pipe. The non-homogenous nature of the material to be discharged results, however, in unreliable operation of the apparatus.
The object of the present invention is to provide a simple and reliable device for discharging ash and other coarse, mainly noncombustible material from the lower part of a reactor.
Another object of the present invention is to provide a discharging device which enables discharging of coarse material with minimized losses in still reactive carbonaceous material.
Still another object of the present invention is to provide a discharging device which enables the recovery of heat from discharged coarse material.
The apparatus according to one embodiment of the invention comprises a substantially J-shaped discharge device, which includes a vertical discharge pipe portion connected with the outlet opening in grid, an upwardly directed discharge opening positioned beneath the outlet opening in the air chamber below the grid, and an aeration space or chamber connecting the vertical pipe portion and the discharge opening. The device further comprises a mechanism for feeding pressurized air in short cycles upwardly into the aeration chamber.
In a second embodiment of the invention, an L-shaped discharge device is connected to the outlet opening such that the coarse material is discharged laterally into the air chamber below the grid. Pressurized air is also introduced laterally, in the direction of the discharge opening, into the aeration chamber connecting the vertical portion with the discharge opening.
In a third embodiment of the invention, a generally L-shaped discharge device is connected to the outlet opening in the grid, wherein the aeration chamber connecting the vertical discharge portion and the discharge opening extends upwardly and away from the vertical portion. Highly pressurized air is supplied, in the form of instantaneous blasts, into the aeration chamber in the direction of the discharge opening.
In a fourth, preferred embodiment, the aeration chamber, which connects the vertical discharge pipe and the discharge opening, extends away from the vertical pipe in opposite directions, and is inclined relative to horizontal. In other words, the discharge device itself has a shape substantially in the form of an upside down T with an inclined crossbar. As in the previously described embodiment, the aeration chamber is inclined so that the end containing the discharge opening is higher than the other, or lower end. The extended lower end of the inclined aeration chamber increases the volume of coarse material which can be discharged with a single air blast. Preferably, the aeration chamber as a whole should be able to hold the same quantity of coarse material as the vertical discharge pipe.
Again, highly pressurized bursts of air are directed into the aeration chamber from the lower end of the aeration chamber and directed toward the discharge opening. By this arrangement, when the aeration chamber has been emptied by a blast of air, new coarse material flows firstly into the lower portion of the chamber and thereafter, into the upper portion of the chamber, provided the aeration chamber is not too steeply inclined. The angle of inclination of the aeration chamber relative to horizontal has to be large enough to hold an optimum amount of coarse material in the upper portion partly to inhibit air from the air chamber (which is at a higher pressure) from flowing into the discharge device. Coarse material in the upper portion of the aeration chamber serves to block this reverse flow of air.
The inclined aeration chamber also poses little risk of coarse material being stuck at the upper or discharge end. Moreover, a baffle which normally closes the discharge opening will remain in a normally closed position due to its own weight, and the higher pressure in the air chamber assists in this regard. To even further ensure that the baffle will remain closed during the lull between air blasts, the weight of the baffle may be increased.
In the preferred embodiment, fluidizing air is added at spaced locations along the vertical pipe portion of the discharge device, as well as in the inclined aeration chamber. The addition of fluidizing air, at very low pressure, removes the finest of particles (those not to be discharged) and returns them to the reactor. It is desirable, of course, to return this fine material to the reactor since it contains combustible carbonaceous material. Best results have been obtained with fluidizing air added in at least the lowermost portions of the vertical pipe and in the aeration chamber. It will be further appreciated that the fluidizing air may be used to cool the discharge pipe and/or aeration chamber.
Preferably, the air pulse pressure in each embodiment is in a range between 2.5 and 7 bar. This range is effective because even the very coarse material is easily discharged by air at the upper end of the pressure range (without negative effects on reactor performance or undue wear). At the same time, the pressure should not be lower than abort 2.5 bar to ensure discharge of coarse material and not primarily the fine materials which would be discharged at very low pressures.
It is also preferred that the air pulses or blasts have a duration of about 30-100 milliseconds. These very short high pressure pulses or blasts need only about 10 liter of air/gas to discharge the material from the aeration chamber into the air chamber. This is advantageous since the small additional quantities of air entering the air chamber have no detrimental effects on the flow of air from the air chamber through the grid into the fluidized bed chamber, nor on the heat exchange normally occurring in the air chamber.
The most significant advantage of the present invention compared to known applications is that, essentially, it does not employ any members moving in the material flow to be discharged.
Other significant advantages are that
the discharge device per se is relatively inexpensive;
it is possible to have a number of discharge devices installed in different configurations depending on the particular circumstances;
the discharge device is easily and quickly installed in various reactor designs, including older combustors;
the discharge device minimizes losses of carbonaceous material discharged along with the coarse material; and
only minimum amounts of air are needed to blast the coarse material out of the aeration chamber, which, in turn, minimizes negative effects on boiler efficiency.
Other objects and advantages will become apparent from the detailed description which follows.